1. Field of the Invention
The present invention relates to a ring trip circuit.
A ring trip circuit is employed as, for example, a part of each subscriber line circuit in a telephone exchange system. When a subscriber is called up by ringing the bell of his telephone set, the ring trip circuit functions, if the subscriber removes the handset from the telephone set, i.e., the telephone is put in an off-hook state, to stop the ringing immediately after the detection of the off-hook state.
2. Description of the Related Art
The prior art ring trip circuit has as one of its essential elements a low-pass filter which cancels an ac component of the ringing tone signal and extracts only a dc component therefrom. However, the use of the low-pass filter produces a major problem that it takes a long time to finally detect a level change of the dc component, this level change occurring in response to a change in state to the off-hook state. This long response time is caused by a first-order response lag inherent to the low-pass filter and usually becomes as long as, e.g., 350 ms. This being so, a ringer stop signal cannot be output until 350 ms lapses after the status change to the off-hook state. Therefore, during the lapse of 350 ms, the called party must unwillingly hear the ringing tone signal from the handset, the sound of this ringing tone signal being generally uncomfortable.
Another problem is that the low-pass filter inevitably requires an amplifier to revive a dc level which has been suppressed together with an ac component when the signal is processed in the low-pass filter.